I was in Washington in July and it rained. When it started, I thought 'Oh good, it'll feel better now.' But no. It's the first time that I've felt hot and unpleasant in a rainstorm. If anything, the humidity got worse.
Incidentally, if you're ever in Washington, go to the Union Street Station. I can honestly say I have never seen a better station anywhere. Americans always tell me that Europe has more history. I've been to 5 or 6 countries in Europe and there isn't a station in any of those countries to compare with Washinton D.C.'s station.
In case anyone's interested in the station: